Gas turbines have a compressor assembly, a combustor assembly and a turbine assembly. The compressor compresses normally ambient air, which is then channeled into the combustor, where it is mixed with a fuel. The fuel and compressed air mixture is ignited, creating a working gas that may reach high temperatures, up to 1300° C. to 1600° C., for example. This working gas then passes through the turbine assembly. In some gas turbines CO2 is the main component of the working medium. In that case pure oxygen is added as is fuel in the combustion chamber to burn and heat up the CO2 gas. The turbine assembly has a rotating shaft holding a plurality of rows of rotating wheels. The turbine assembly can have a plurality of stationary wheels attached to a casing of the turbine. Each rotating wheel is preceded by a stationary wheel to direct the working gas at an optimum angle against the vanes of the rotating wheels. Expansion of the working gas through the turbine assembly results in a transfer of energy from the working gas to the rotating wheels, causing rotation of the shaft.
Each vane of a wheel may have an outer platform connected to a radially outer end of the vane body for attachment to the turbine casing, and an inner platform connected to the inner end of the vane body. The outer platforms for a given row of vanes are mounted adjacent to each other as segments in a circular array, defining an outer shroud ring. The inner platforms are likewise mounted adjacent to each other in a circular array, defining an inner shroud ring. These outer and inner shroud rings define a flow channel between them to channel the working gas.
The vane body may include passages for a cooling fluid, such as air. However, the surfaces of the vane assemblies exposed to the working gas are subjected to high operational temperatures and thermal stresses. This can cause cracks in the vane body and platforms. Typically, each vane body and at least one platform are formed together as a unitary structure, so damage to a platform may require replacement of an entire vane assembly, even when the vane body is still in a serviceable condition.
Each vane of a turbine engine, like a gas or steam turbine engine, has areas of excessive stress in the aerofoil leading edge area due to a mechanical loading of the vane in the downstream direction. Such vanes have an internal cooling and therefore and because of thermal stress reasons the vane body of the vanes has a limit on the maximum wall thickness.
The aerodynamic design of the vane body has been changed in the past to give a larger volume of material at the leading edge whilst maintaining the maximum wall thickness. Therefore the aerodynamic performance is degraded in order to reduce the stress levels to an acceptance limit.
To improve the strength of the vane body several constructive features are known. The U.S. Pat. No. 5,484,258 discloses a guide vane with a double outer wall. The outer wall of the vane body has a one-pieced integrally formed double wall construction including an inner wall spaced apart from an outer wall with mechanically and thermally tying elements in the form of continuous tying ribs which are integrally formed with and disposed between the inner and outer walls. The ribs space apart the inner and outer walls and respectively such that the walls are essentially parallel to each other. Such a double outer wall is structurally very complicated and expensive to manufacture.
A web-like structure in the inside of a vane body is known from U.S. Pat. No. 5,660,524. The vane body has a first outer wall and a second outer wall together defining an airfoil shape including a leading edge, a trailing edge, a pressure side along the first outer wall, a suction side along the second outer wall, a blade tip and a blade root. Between the two outer walls are a couple of monolithic inner walls arranged. These monolithic inner walls have a web-like structure to strengthen the vane body or the outer walls of the vane body, respectively. The web-like inner structure makes the cooling of the vane body complicated and expensive. Further, the web-like inner structure increases the weight of the vane and therefore decreases the aerodynamic performance of the vane.
A similar web-like structure is known from U.S. Pat. No. 2,974,926. The turbine blade comprises an outer shell and a center strut-root-fin assembly. The center strut-root-fin assembly comprises a root, which is preferably of a conventional fir tree type. The root extends downwardly from a root platform and is embedded in the core of a turbine motor. The center strut-root-fin assembly further comprises a strut to which fins are attached. The strut is secured to the root platform. The strut extends from the root platform upwardly. In the assembled position of the turbine blade the outer shell of the turbine blade is slipped over the strut-root-fin assembly. The outer shell is spot welded at various places along the blade height to the fins of the strut-root-fin assembly and at various places around a shell platform to a root platform ledge. The spot welds between the fins of the strut-root-fin assembly and the outer shell are arranged at the suction side and at the pressure side of the turbine blade. To enable to slip the outer shell over the strut-root-fin assembly there are tolerances between the outer shell and the strut-root-fin assembly. Welding the fins to the suction side and the pressure side of the outer shell is complicated and cost effective.
US 2009/0047136 A1 discloses a stator with an airfoil, an outer diameter shroud and an inner diameter shroud. The airfoil comprises a thin-walled structure that forms a hollow cavity leading edge, pressure side, suction side and trailing edge. The airfoil further comprises interior cooling features including cooling channels, cooling holes, gill holes, peanut cavities, an impingement rib, nozzles, a divider, partitions, trip strips, an outer diameter end cap and an inner diameter end cap. Cooling air enters the cooling holes at the leading edge of the airfoil to flow into the peanut cavity between the leading edge and the impingement rib or into to cooling channels behind the impingement rib. The impingement rib is spaced apart from the leading edge and runs partially parallel to the leading edge of the airfoil.